The most efficient protection against influenza infection is vaccination against circulating strains and it is important to produce influenza viruses for vaccine production as quickly as possible.
Wild-type influenza viruses often grow to low titres in eggs and cell culture. In order to obtain a better-growing virus strain for vaccine production it is possible to reassort the circulating vaccine strain with a faster-growing high-yield donor strain. This can be achieved by co-infecting a culture host with the circulating influenza strain and the high-yield donor strain and selecting for reassortant viruses which contain the hemagglutinin (HA) and neuraminidase (NA) segments from the vaccine strain and the other viral segments (i.e. those encoding PB1, PB2, PA, NP, M1, M2, NS1 and NS2) from the donor strain. Another approach is to reassort the influenza viruses by reverse genetics (see, for example references 1 and 2).
Whilst it is common practice to use reassortant influenza A strains in vaccine production, reassortant influenza B strains are not usually used because wild-type influenza B viruses usually provide adequate yields in eggs. Furthermore, wild-type influenza B viruses have been reported to have a growth advantage over reassortant influenza B viruses (see, for example, reference 3). Accordingly, high growth influenza B reassortants have been generated only for a small number of recent influenza B viruses. These reassortants typically contain a mixture of backbone gene segments derived from B/Lee/40, B/Brisbane/60/08 and B/Panama/45/90 (4, 5).
To date, only two reassortant influenza B viruses (BX-35 and BX-39) have been used for commercial vaccine manufacturing. BX-35 contains the HA, NA, PA, PB1, and NS segments from the B/Brisbane/60/08 strain, the PB2 and M segments from B/Panama/45/90, and the NP segment from B/Lee/40. BX-39 contains the HA, NA, PBI, and M segments from the circulating B/Hubei-Wujiagang/159/08 strain, the PA and NS segments from B/Panama/45/90, and the PB2 and NP segments from B/Lee/40(6, 7).
There are currently only a limited number of donor strains for reassorting influenza B viruses for vaccine manufacture and the known reassortant influenza B viruses do not always grow better than the parent strain. Thus, there is a need in the art to provide further and improved donor strains for influenza B virus reassortment.